For decades, scientists believed dogs could only see in monochrome and used brightness levels - whether something looked lighter or darker next to another object - to identify outlines of items.
However, last year scientist Jay Neitz from the University of Washington, carried out experiments on dogs to test this theory.
Human eyes have three 'cones' that detect colour and can identify red, blue, green and yellow wavelengths created by light entering the eye.
Neitz discovered that dogs only have two cones - this means they can distinguish blue and yellow, but not red and green.
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